Phenylketonuria (PKU) is an autosomal recessive genetic disorder where the majority of cases are caused by mutations in the phenylalanine hydroxylase (PAH) gene. The PAH gene encodes a hepatic enzyme that catalyzes the hydroxylation of L-phenylalanine (Phe) to L-tyrosine (Tyr) upon multimerization. Reduction or loss of PAH activity leads to phenylalanine accumulation and its conversion into phenylpyruvate (also known as phenylketone). This abnormality in phenylalanine metabolism impairs neuronal maturation and the synthesis of myelin, resulting in mental retardation, seizures and other serious medical problems.
Currently, there is no cure for PKU. The standard of care is diet management by minimizing foods that contain high amounts of phenylalanine. Dietary management from birth with a low phenylalanine formula largely prevents the development of the intellectual disability of the disorder. However, even on a low-phenylalanine diet, children still suffer from growth retardation, and adults often have osteoporosis and vitamin deficiencies. Moreover, adherence to life-long dietary treatment is difficult, particularly once children reach school age.
New treatment strategies have recently emerged, including large neutral amino acid (LNAA) supplementation, cofactor tetrahydrobiopterin therapy, enzyme replacement therapy, and genetically modified probiotic therapy. However, these strategies suffer from shortcomings. The LNAA supplementation is suitable only for adults not adhering to a low Phe diet. The cofactor tetrahydrobiopterin can only be used in some mild forms of PKU. Enzyme replacement by administration of a substitute for PAH, e.g., phenylalanine ammonia-lyase (PAL), can lead to immune responses that reduce the efficacy and/or cause side effects. As to genetically modified probiotic therapy, the pathogenicity of PAL-expressing E. coli has been a concern.
Gene therapy provides a unique opportunity to cure PKU. Retroviral vectors, including lentiviral vectors, are capable of integrating nucleic acids into host cell genomes, raising safety concerns due to their non-targeted insertion into the genome. For example, there is a risk of the vector disrupting a tumor suppressor gene or activating an oncogene, thereby causing a malignancy. Indeed, in a clinical trial for treating X-linked severe combined immunodeficiency (SCID) by transducing CD34+ bone marrow precursors with a gammaretroviral vector, four out of ten patients developed leukemia (Hacein-Bey-Abina et al., J Clin Invest. (2008) 118(9):3132-42). Non-integrating vectors, on the other hand, often suffer insufficient expression level or inadequate duration of expression in vivo.
Accordingly, there is a need in the art for improved gene therapy compositions and methods that can efficiently and safely restore PAH gene function in PKU patients.